Novel Ring Filters As Wide-Band 180° Transmission Lines

A novel ring fiter as a wide-band 180° transmission line is introduced, designed and analyzed. Most of conventional ring filters have used gaps between a ring and the feeding lines as a coupling element Due to the gap discontinuities, the circuits realized with a ring together with feeding lines suffer from high-insertion lost To reinforce this weak point, two feeding lines are directly connected at the 0° and 180° points and two ¿g/4 stubs are short-circuited at the 90° and 270° point of the ring. These two stubs are necessary to reject the power at even multiples of the design center frequency and as resonators for a filter characteristics. As the ring filter is designed with the concept of power division at the input port and power combining at the output port, two different parts of the combined power have the same phases in the whole frequency range if the transmission lines are ideal. Therefore, simulation performances are quite good with an insertion loss of ¿0.034 dB and a return loss less than ¿20 dB in a relative bandwidth of about 2.2: 1. Also, it is shown that this ring filter can be used as a wide-band impedance transformer.